Size: 11 to 15 feet from nose to tail.
Appearance: That of a typical alligator, though there are some reports of Albino specimens being seen or captured.
Threat: Medium. They are known to attack, kill, and even eat humans when the opportunity presents itself, but they don't go out of their way to track or attack us.
It's the location of the sightings which helps this critter make the list. Reports of alligators being found in sewers and storm drains in major cities like New York and Chicago, places well outside the American Alligator's range. Fortunately, though, they are limited to the largest and oldest of American cities, ones with the old large and extensive tunnel systems needed to service cities of their sizes.
How they get there isn't much of a mystery either. Most are simple animals which were illegally obtained as hatchlings who were dumped into the sewer or flushed down the toilet when they started getting too big for their owner's or when their owner's just got bored of them. Some people have suggests, as well, that with summers getting hotter, Alligators have been expanding their range, swimming up the rivers and the coast, and finding entrances into the sewers which provide them shelter when the temperature starts to drop again.
There are people that belittle the idea of alligators in sewers though, claiming hoaxes and plants for publicity stunts. They point out the lack of decent food sources and the temperature of the sewers as reasons cold-blooded animals like Alligators couldn't survive in such places. Unfortunately, neither of these arguments hold up to any scrutiny. Anyone who has been to one of these types of cities is well aware of the problem with rats, which would provide small to medium alligators with an ample food supply, not to mention any food thrown out by humans which then gets washed down.
The temperature isn't actually an issue either. It used to be thought that it would be too cold down there for such animals to survive, however, recent investigations have determined the underground levels of most large cities actually maintains and steady temperature almost year round. The dark caverns are insulated from the weather above. An investigation by the show Monsterquest found in New York, even in the middle of winter, with snow falling, had areas of the sewers which maintained a cozy 68 degrees. Ask anyone who has looked into building a house underground and they'll probably be more than happy to tell you what a great natural insulator the earth is.
The only real issue, is that there isn't enough material in the sewers for them to build nests, so most likely, while one or two might find their way in and set up house, you're not going to find a breeding population. Which explains why the discovery of such creatures remains rare.
The idea of such large, predatory creatures being right under our feet has inspired a number of movies, most notably the series of movies titled Alligator. It's interesting to note that while a lot of people have trouble differentiating between an alligator and a crocodile, Hollywood doesn't seem to have such an issue, with alligators being city-dwellers while anything actually in a swamp is a crocodile. Of course though, this doesn't seem to matter between the animal actors in such movies.
Even in cities, it's not a bad idea to keep in mind what could be right underneath your feet. Really starting to feel like there's no place safe from monsters, isn't it?